DEPENDENCE OF DIFFUSIVE PERMEATION RATES ON UPSTREAM AND DOWNSTREAM PRESSURES .3. MEMBRANE SELECTIVITY AND IMPLICATIONS FOR SEPARATION PROCESSES

In previous papers we have shown that a semi-empirical thermodynamic-diffusive model appears to duplicate the behavior of the hexane/heptane/polyethylene system with regard to the effect of upstream and downstream pressure on rate and separation. In the present paper, we generalize this model, and discuss its implications for the use of membranes in separation processes. In particular, we show that increasing the downstream pressure can either increase or decrease the concentration of the more volatile component in the permeate depending upon the membrane selectivity. In addition, it is suggested that for systems in which pervaporation is superior to partial vaporization, including azeotropic mixtures, the best separations will be achieved with high vacuum downstream. Finally, the processes of reverse osmosis and pervaporation are compared. © 1978.